Possible to activate different object shape keys at same time? Regarding .vrm --> .fbx model for vrchat & vtuber programs
Not quite the answer I was looking for, but I've figured out a way to get things to work once exported.
You do not need to be able to activate them at the same time within blender, you just need to make sure that all of them are named the same thing, and that you can re-export your .FBX file to a .VRM file once imported to unity. This seems to be the only way to get your shapes/expressions to work properly if they are not all within one object.
Things needed:

CATS extension for blender https://github.com/GiveMeAllYourCats/cats-blender-plugin
Unity VRM export/importer [Apologies I forgot where I got this from so will update with link later]


First things first, This works better if your objects are linked and you've assigned parent and child bones. [How to parent/child bones: https://docs.blender.org/manual/en/2.79/editors/3dview/object/properties/relations/parents.html ]
You want to make sure that all of your shape keys that &quot;connect&quot; and you want to go together are named the same thing and line up properly. Both for ease of access and to make sure things export/import correctly for blendshapes in unity. Then you want to &quot;Fix&quot; your model with the CATS extension, under the &quot;Model&quot; category. All default settings should be fine except for the &quot;Join Meshes&quot; option. Because my mesh and objects are separated with their own keyshapes, joining the meshes here will get rid of or ruin the shapes.

Once your model is fixed, check your shape keys to see if any of them got altered. At this point you are free to go through and delete any shape keys that were transferred over when you separated your mesh/objects with your original vroid model, that are unneeded or do not affect your object. If everything is fine and once you've removed or added any extra shapes you need, then you want to export your model to a .fbx file. Make sure to select the embed textures option under settings in the CATS menu.
I'm going to skip over a bit of this final process as it's regarding unity entirely at this point, but essentially you want to import your .fbx model, make sure all the textures and materials are correct, and then you want to export your model to a .vrm model within unity.
From there you should be able to find a folder titled blendshapes and be able to alter the animations inside with sliders to create a &quot;full&quot; facial expression, pose, etc, as long as you exported and fixed everything correctly within blender.